1. Field of the Invention
The present invention is directed in general to the field of semiconductor devices. In one aspect, the present invention relates to high-performance complementary metal oxide semiconductor (CMOS) field effect transistors (FETs) fabricated on hybrid substrates.
2. Description of the Related Art
CMOS devices, such as NMOS or PMOS transistors, have conventionally been fabricated on semiconductor wafers that have a single crystal orientation, such as Si having a (100) crystal orientation. Those skilled in the art have discovered that electron mobility for NMOS devices is highest when such NMOS devices are built on a (100) surface-oriented substrate, but the hole mobility for PMOS devices built on a (100) substrate is fairly degraded. That is, hole mobility values on (100) Si are roughly 2×-4× lower than the corresponding electron mobility for this crystallographic orientation. On the other hand, it is also known that hole mobility for PMOS devices is more than doubled on a (110) surface-oriented substrate, while the electron mobility for NMOS devices built is degraded with this substrate orientation. As a result, PMOS devices formed on a (110) surface will exhibit significantly higher drive currents than PMOS devices formed on a (100) surface. Prior attempts to address this problem have attempted to form hybrid substrates with different surface orientations using semiconductor-on-insulator (SOI) wafer bonding to provide PMOS and NMOS devices with their own optimized crystal orientation.
Additional challenges are posed by the ever shrinking size and scaling of semiconductor device technology. For example, gate leakage current due to direct tunneling through thinner silicon oxide gate layers is an additional problem with smaller devices. An additional challenge posed by decreasing feature sizes is that aspects of device design and fabrication that previously gave rise to only second-order effects in long-channel devices can no longer be ignored. For example, the scaling of channel length and gate oxide thickness in a conventional MOS transistor exacerbates problems of polysilicon gate depletion, high gate resistance, leakage current and dopant (i.e., boron) penetration into the channel region of the device.
Accordingly, there is a need for improved semiconductor processes and devices to overcome the problems in the art, such as outlined above. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposes of promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.